The filter design plays an important role in the communication system. An ideal received filter can minimize the distortion of the signals and filter the interference out of band to achieve the maximum signal-to-noise ratio (SNR), and an acceptable bandwidth-limitation. However, the purpose of minimizing the distortion exhibits some conflicts with that of filtering the noise and the interference completely. Therefore, design of the received filters has to make a balance between the distortion and the noise.
On the other hand, the transmitted filter is used to make the bandwidth of the transmitted signal satisfy the standard in the specific spectrum. Because the transmitted signal for the digital communication is generated from a digital-to-analog converter, the transmitted filter should be designed to remove the harmonic which is generated from the operation of sample and hold of the digital-to-analog converter. In the design of the transmitted filter, the requirements for removing harmonic are decided according to the characteristics of the digital-to-analog converter, such as sampling frequency. Due to the rebuilding operation of the transmitted filter for the analog waveform (trapezoid) of the digital-to-analog converter, the transmitted filter is named as rebuilding filter as well.
Bandwidth is the most parameter for the filter design. In the conventional communication system, the bandwidth of the transmitted filter is larger than the bandwidth of the received filter. Therefore, in order to optimize the efficiency of the received filter and the transmitted filter, separable transmitted filter and separable received filter are adopted in the design of the recent transceiver. However, due to the independence of the separable transmitted filter and the separable received filter, more time is spent to switch the transmitted mode and the received mode, and the size of the transmitter and the cost of the products are also increased.
FIG. 1 shows a common system of the filtering circuit of the transmitted port and the received port in the prior invention. The filtering circuit switches the transmitted mode and the received mode to minimize circuit size through the plural switches. However, the filtering circuit is not suitable for the distinct systems.
U.S. Pat. No. 2007/0040605 A1 discloses a shared receiver and transmitter filter. In order to share the filters, the invention adopts four switches into the received circuit and the transmitted circuit to switch the transmitted mode and the received mode. However, the disclosed filter is merely suitable for the common system of the circuit of the transmitted port and the received port.
According to the above discussions, it need an apparatus and a method to overcome the disadvantage of the prior art.